Direct observation of non-fully-symmetric coherent optical phonons by femtosecond x-ray diffraction

Abstract

We directly measure by femtosecond time-resolved x-ray diffraction the $E$${}_{g}$ symmetry coherent phonon excited in bismuth by a strong optical pulse. The magnitude of the $E$${}_{g}$ mode observed is 0.2 pm peak-to-peak, compared against the $2.7$-pm initial displacement of the fully symmetric ${A}_{1g}$ mode. The much smaller motion of the $E$${}_{g}$ mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. The experimentally measured magnitude of the $E$${}_{g}$ motion allows us to rule out a previously suggested scenario for explaining the dynamics in bismuth that relies on strong coupling between these modes.